Thirty children with refractory acute lymphocytic leukemia (ALL) were treated with mitoxantrone, 8 mg/m2/day, for 5 days. Three children received a second course of the drug 3 to 4 weeks later. All but two patients had received prior anthracycline therapy. There were 2 complete responses, 4 early deaths, and 24 patients with persistent leukemia. Of the 21 patients with circulating blasts at the start of mitoxantrone who did not achieve remission, 16 (76%) had complete clearance of their peripheral blood blasts. Although all patients developed profound neutropenia (less than 100 per mm-3), mucosal and hepatic toxicities were uncommon and mild. Mitoxantrone has moderate activity in childhood ALL and should be considered for further trials in less heavily pretreated patients.